1. Field of the Invention
The present invention relates to a nonwoven fabric comprising a melt-stable, biodegradable lactide polymer composition and a process for manufacturing such nonwoven fabrics from a melt-stable, biodegradable lactide polymer.
2. Description of the Prior Art
The need for and uses of nonwoven fabrics have increased tremendously in recent years. Production of nonwoven roll goods was estimated at 2.5 billion pounds in 1992. Nonwoven fabrics are presently used for coverstock, interlinings, wipes, carrier sheets, furniture and bedding construction, filtration, apparel, insulation, oil cleanup products, cable insulating products, hospital drapes and gowns, battery separators, outerwear construction, diapers and feminine hygiene products.
There are basically three different manufacturing industries which make nonwovens; the textile, paper and extrusion industries. The textile industry garnets, cards or aerodynamically forms textile fibers into oriented webs. The paper industry employs technology for converting dry laid pulp and wet laid paper systems into nonwoven fabrics. The extrusion industry uses at least three methods of nonwoven manufacture, those being the spunbond, melt blown and porous film methods. The melt blown method involves extruding a thermoplastic resin through a needle thin die, exposing the extruded fiber to a jet of hot air and depositing the xe2x80x9cblownxe2x80x9d fiber on a conveyor belt. These fibers are randomly orientated to form a web. The spunbond method also utilizes a needle thin die, but orients or separates the fibers in some manner. The porous film method employs both slit and annular dies. In one method, a sheet is extruded and drawn, fibrillization occurs and a net-like fabric results.
A problem associated with current nonwoven materials is that recycling of the article containing the nonwoven fabric is generally not cost effective. In addition, disposal generally involves creating non-degradable waste. A vivid example is the disposal of diapers. Disposable diapers rely heavily on the use of nonwovens in their construction. Millions of diapers are disposed of each year. These disposable diapers end up in landfills or compost sites. The public is becoming increasingly alarmed over diapers that are not constructed of biodegradable material. In order to address the public""s concern over the environment, diaper manufacturers are turning to biodegradable materials for use in their diapers. Currently, biodegradable materials made from starch based polymers, polycaprolactones, polyvinyl alcohols, and polyhydroxybutyrate-valerate-copolymers are under consideration for a variety of different uses in the disposable article market. However, to date, there has not been a satisfactory nonwoven fabric made from a biodegradable material which has properties that can withstand the present requirements of nonwoven fabrics. Although not believed to be known as a precursor for nonwoven fabric, the use of lactic acid and lactide to manufacture a biodegradable polymer is known in the medical industry. As disclosed by Nieuwenhuis et al. (U.S. Pat. No. 5,053,485), such polymers have been used for making biodegradable sutures, clamps, bone plates and biologically active controlled release devices. Processes developed for the manufacture of polymers to be utilized in the medical industry have incorporated techniques which respond to the need for high purity and biocompatability in the final product. These processes were designed to produce small volumes of high dollar-value products, with less emphasis on manufacturing cost and yield.
In order to meet projected needs for biodegradable packaging materials, others have endeavored to optimize lactide polymer processing systems. Gruber et al. (U.S. Pat. No. 5,142,023) disclose a continuous process for the manufacture of lactide polymers with controlled optical purity from lactic acid having physical properties suitable for replacing present petrochemical-based polymers.
Generally, manufacturers of polymers utilizing processes such as those disclosed by Gruber et al. will convert raw material monomers into polymer beads, resins or other pelletized or powdered products. The polymer in this form may then be then sold to end users who convert, i.e., extrude, blow-mold, cast films, blow films, thermoform, injection-mold or fiber-spin the polymer at elevated temperatures to form useful articles. The above processes are collectively referred to as melt-processing. Polymers produced by processes such as those disclosed by Gruber et al., which are to be sold commercially as beads, resins, powders or other non-finished solid forms are generally referred to collectively as polymer resins.
Prior to the present invention, it is believed that there has been no disclosure of a combination of composition control and melt stability requirements which will lead to the production of commercially viable lactide polymer nonwoven fabrics.
It is generally known that lactide polymers or poly(lactide) are unstable. The concept of instability has both negative and positive aspects. A positive aspect is the biodegradation or other forms of degradation which occur when lactide polymers or articles manufactured from lactide polymers are discarded or composted after completing their useful life. A negative aspect of such instability is the degradation of lactide polymers during processing at elevated temperatures as, for example, during melt-processing by end-user purchasers of polymer resins. Thus, the same properties that make lactide polymers desirable as replacements for non-degradable petrochemical polymers also create undesirable effects during processing which must be overcome.
Lactide polymer degradation at elevated temperature has been the subject of several studies, including: I. C. McNeill and H. A. Leiper, Polymer Degradation and Stability, vol. 11, pp. 267-285 (1985); I. C. McNeill and H. A. Leiper, Polymer Degradation and Stability, vol. 11, pp. 309-326 (1985); M. C. Gupta and V. G. Deshmukh, Colloid and Polymer Science, vol. 260, pp. 308-311 (1982); M. C. Gupta and V. G. Deshmukh, Colloid and Polymer Science, vol. 260, pp. 514-517 (1982); Ingo Luderwald, Dev. Polymer Degradation, vol. 2, pp. 77-98 (1979); Domenico Garozzo, Mario Giuffrida, and Giorgio Montaudo, Macromolecules, vol. 19, pp. 1643-1649 (1986); and, K. Jamshidi, S. H. Hyon and Y. Ikada, Polymer, vol. 29, pp. 2229-2234 (1988).
It is known that lactide polymers exhibit an equilibrium relationship with lactide as represented by the reaction below: 
No consensus has been reached as to what the primary degradation pathways are at elevated processing temperatures. One of the proposed reaction pathways includes the reaction of a hydroxyl end group in a xe2x80x9cback-bitingxe2x80x9d reaction to form lactide. This equilibrium reaction is illustrated above. Other proposed reaction pathways include: reaction of the hydroxyl end group in a xe2x80x9cback-bitingxe2x80x9d reaction to form cyclic oligomers, chain scission through hydrolysis of the ester bonds, an intramolecular beta-elimination reaction producing a new acid end group and an unsaturated carbon-carbon bond, and radical chain decomposition reactions. Regardless of the mechanism or mechanisms involved, the fact that substantial degradation occurs at elevated temperatures, such as those used by melt-processors, creates an obstacle to use of lactide polymers as a replacement for petrochemical-based polymers. It is apparent that degradation of the polymer during melt-processing must be reduced to a commercially acceptable rate while the polymer maintains the qualities of biodegradation or compostability which make it so desirable. It is believed this problem has not been addressed prior to the developments disclosed herein.
As indicated above, poly(lactide)s have been produced in the past, but primarily for use in medical devices. These polymers exhibit biodegradability, but also a more stringent requirement of being bioresorbable or biocompatible. As disclosed by M. Vert, Die Ingwandte Makromolekulare Chemie, vol. 166-167, pp. 155-168 (1989), xe2x80x9cThe use of additives is precluded because they can leach out easily in body fluids and then be recognized as toxic, or, at least, they can be the source of fast aging with loss of the properties which motivated their use. Therefore, it is much more suitable to achieve property adjustment through chemical or physical structure factors, even if aging is still a problem.xe2x80x9d Thus, work aimed at the bioresorbable or biocompatible market focused on poly(lactide) and blends which did not include any additives.
Other disclosures in the medical area include Nieuwenhuis (European Patent No. 0 314 245), Nieuwenhuis (U.S. Pat. No. 5,053,485), Eitenmuller (U.S. Pat. No. 5,108,399), Shinoda (U.S. Pat. No. 5,041,529), Fouty (Canadian Pat. No. 808,731), Fouty (Canadian Pat. No. 923,245), Schneider (Canadian Patent No. 863,673), and Nakamura et al., Bio. Materials and Clinical Applications, Vol. 7, p. 759 (1987). As disclosed in these references, in the high value, low volume medical specialty market, poly(lactide) or lactide polymers and copolymers can be given the required physical properties by generating lactide of very high purity by means of such methods as solvent extraction or recrystallization followed by polymerization. The polymer generated from this high purity lactide is a very high molecular weight product which will retain its physical properties even if substantial degradation occurs and the molecular weight drops significantly during processing. Also, the polymer may be precipitated from a solvent in order to remove residual monomer and catalysts. Each of these treatments add stability to the polymer, but clearly at a high cost which would not be feasible for lactide polymer compositions which are to be used to replace inexpensive petrochemical-based polymers in the manufacture of nonwoven products.
Furthermore, it is well-known that an increase in molecular weight generally results in an increase in a polymer""s viscosity. A viscosity which is too high can prevent melt-processing of the polymer due to physical/mechanical limitations of the melt-processing equipment. Melt-processing of higher molecular weight polymers generally requires the use of increased temperatures to sufficiently reduce viscosity so that processing can proceed. However, there is an upper limit to temperatures used during processing. Increased temperatures increase degradation of the lactide polymer, as the previously-cited studies disclose.
Jamshidi et al., Polymer, Vol. 29, pp. 2229-2234 (1988) disclose that the glass transition temperature of a lactide polymer, Tg, plateaus at about 57xc2x0 C. for poly(lactide) having a number average molecular weight of greater than 10,000. It is also disclosed that the melting point, Tm, of poly (L-lactide) levels off at about 184xc2x0 C. for semi-crystalline lactide polymers having a number average molecular weight of about 70,000 or higher. This indicates that at a relatively low molecular weight, at least some physical properties of lactide polymers plateau and remain constant.
Sinclair et al. (U.S. Pat. No. 5,180,765) disclose the use of residual monomer, lactic acid or lactic acid oligomers to plasticize poly(lactide) polymers, with plasticizer levels of 2-60 percent. Loomis (U.S. Pat. No. 5,076,983) discloses a process for manufacturing a self-supporting film in which the oligomers of hydroxy acids are used as plasticizing agents. Loomis and Sinclair et al. disclose that the use of a plasticizer such as lactide or lactic acid is beneficial to produce more flexible materials which are considered to be preferable. Sinclair et al., however, disclose that residual monomer can deposit out on rollers during processing. Loomis also recognizes that excessive levels of plasticizer can cause unevenness in films and may separate and stick to and foul processing equipment. Thus, plasticizing as recommended, negatively impacts melt-processability in certain applications.
Accordingly, a need exists for a lactide polymer composition which is melt-stable under the elevated temperatures common to melt-processing resins in the manufacture of nonwoven fabrics. The needed melt-stable polymer composition must also exhibit sufficient compostability or degradability after its useful life as a nonwoven fabric. Further, the melt-stable polymer must be processable in existing melt-processing equipment, by exhibiting sufficiently low viscosities at melt-processing temperatures while polymer degradation and lactide formation remains below a point of substantial degradation and does not cause excessive fouling of processing equipment. Furthermore, the polymer lactide must retain its molecular weight, viscosity and other physical properties within commercially-acceptable levels through the nonwoven manufacturing process. It will be further appreciated that a need also exists for a process for manufacturing such nonwoven fabrics. The present invention addresses these needs as well as other problems associated with existing lactide polymer compositions and manufacturing processes. The present invention also offers further advantages over the prior art, and solves other problems associated therewith.
According to the present invention, a nonwoven fabric comprising a plurality of fibers is provided. A first portion of the plurality of fibers comprise a melt-stable lactide polymer composition comprising: a plurality of poly(lactide) polymer chains having a number average molecular weight of from about 10,000 to about 300,000; lactide in a concentration of less than about 2 percent by weight; and water in a concentration of less than about 2,000 parts per million. A process for the manufacture of the nonwoven fabric is also provided. For the purposes of the present invention, the nonwoven fabric may be manufactured from any number of methods and is not to be limited by the particular method.
Optionally, stabilizing agents in the form of anti-oxidants and water scavengers may be added. Further, plasticizers, nucleating agents and/or anti-blocking agents may be added. The resultant nonwoven fabric is biodegradable and may be disposed of in an environmentally sound fashion.
Poly(lactide) is a polymeric material which offers unique advantages as a fiber for nonwovens not only in the biodegradable sense, but in the manufacturing process as well.
Poly(lactide) offers advantages in the formation of the nonwoven fabric in a melt extrusion process. One problem that is sometimes encountered in the extrusion of fibers into a nonwoven web is poor adhesion of the fibers to one another upon cooling. Two characteristics of poly(lactides) lend themselves to enhanced adhesion: low viscosity and high polarity. Mechanical adhesion, the interlocking of fibers at adjoining points, increases as the viscosity decreases. An advantage of poly(lactide) is that the viscosity lends itself well to fiber formation. Thus, poly(lactide) fibers adhere to one another well, resulting in a web with added strength. Also, because the surface is typically polar for most fibers, the high polarity of the poly(lactide) offers many dipole-dipole interactions, further resulting in enhanced adhesion.
In melt blown processes, the fibers of the present invention have small diameters which are beneficial for many applications. The present fibers can have diameters of less than about 5 xcexcm.
The fibers of the nonwoven web of the present invention are superior to typical polypropylene nonwoven webs in diaper construction. The typical construction of a diaper comprises an outer, water impervious back sheet, a middle, absorbent layer and an inner layer, which is in contact with the diaper wearer. The inner layer is typically made from a soft, nonwoven material such as a polypropylene nonwoven web. However, polypropylene, due to its low polarity, has to be surface modified such that the urine passes through the inner layer, rather than being repelled. A significant advantage of the present invention is that the polarity of the poly(lactide) (without surface treatment) is ideally suited such that urine readily passes through the nonwoven web, but is not absorbed by the layer. Thus, the poly(lactide) web of the present invention is a superior inner layer for diaper construction. The present invention may also be employed in incontinent and feminine hygiene products.
The nonwoven fabric of the present invention also may be used in packaging and bagging operations. Food packaging which does not require water tight packaging but requires breathability is an example of a use of the present invention. Bags such as leaf or yard bags may also be made from the nonwoven fabric of the present invention. The fabric is porous to allow air to enter the bag to begin decomposing the leaves. This is advantageous over present leaf bags, which do not allow air to penetrate into the leaf cavity. Further, the present nonwoven fabric, when used as a leaf bag, decomposes along with the leaves, thus minimizing the adverse environmental impact of the leaf bags.
Poly(lactide) processes at lower temperatures which allows the fiber to be extruded at lower temperatures than traditional polymers. This results in a cost savings to the converter because the extrusion equipment will not require as much power when run at lower temperatures. There is also increased safety associated with lower temperatures.
A significant advantage of poly(lactide) over many nonwoven fabrics used today such as polypropylene is its biodegradability. The continued depletion of landfill space and the problems associated with incineration of waste have led to the need for development of a truly biodegradable nonwoven fabric to be utilized as a substitute for non-biodegradable or partially biodegradable petrochemical-based nonwoven fabrics. Furthermore, a poly(lactide) nonwoven web, unlike other biodegradable polymers, is believed to not support microbial growth. Starch or other biodegradable polymers, when exposed to warm, damp environments, will promote the growth of unhealthy microbes. This is undesirable in the diaper industry. Thus the present invention has yet another advantage over prior biodegradable polymers.
The above described features and advantages along with various other advantages and features of novelty are pointed out with particularity in the claims of the present application. However, for a better understanding of the invention, its advantages, and objects attained by its use, reference should be made to the drawings which form a further part of the present application and to the accompanying descriptive matter in which there is illustrated and described preferred embodiments of the present invention.